In general, the human visual system is formed of two major processing pathways referred to as the magnocellular pathway and the parvocellular pathway. These two subdivisions or pathways remain largely segregated and independent throughout the visual system. The two pathways begin in the retina but are most apparent in the lateral geniculate nucleus (LGN). In the LGN, cells in the ventral or magnocellular layers are larger than cells in the dorsal or parvocellular layers. In the retina and LGN, the magno and parvo subdivisions differ physiologically in four major ways: color selectivity, contrast sensitivity, temporal resolution, and acuity.
In particular, fast, low contrast visual information is carried by the magnocellular subdivision or pathway, and slow high contrast information is carried by the parvocellular subdivision/pathway. This functional segregation, begun in the retina, continues throughout the visual system, possibly even up through higher cortical association areas. Therefore, a problem specific to the magnocellular pathway could originate at any level from the retina to prestriate visual cortical areas, and it would be difficult, using behavioral tests, to localize such perceptual defects.
For example, development dyslexia is the selective impairment of reading skills despite normal intelligence, sensory acuity, motivation and instruction. Several perceptual studies have suggested that dyslexic subjects process visual information more slowly than normal subjects. The flicker fusion rate, which is the fastest rate at which a contrast reversal of a stimulus can be seen, is abnormally slow in dyslexic children at low spatial frequencies and low contrast. When two visual stimuli are presented in rapid succession, the two images fuse and appear as a single presentation. The temporal separation necessary to distinguish two presentations measures visual persistence, and for dyslexic children the temporal separation is a hundred milliseconds longer than for normal children, particularly for low spatial frequency stimuli. Dyslexic subjects also have trouble distinguishing the order of two rapidly flashed visual stimuli. In contrast, dyslexics perform normally on test having prolonged stimulus presentations.
Thus, past perceptual studies suggest that the fast functioning part of the visual system is slowed down in dyslexia. Tests using behavioral methods have been unable to more specifically detect or localize the perceptual defects involved in dyslexia. On the other hand, further studies have been made to more accurately determine dyslexia using visually evoked potentials in response to stimuli at low spatial frequency (see "Factor Scores Derived From Visual Evoked Potential Latencies Differentiate Good and Poor Readers", by James G. May et al., Clin. Vision Sci., Vol. 7, No. 1, pages 67-70, 1992) and visually evoked potentials in response to background flicker (see "The Effects of Uniform Field Flicker on Visual Evoked Potentials in Children With Reading Disability" by Steven Lehmkuhle et al., Investigative Opthomology and Visual Science, Supplement, Vol. 33 (1992), page 718.